Electroless deposition of metal is effected by immersing catalyzed substrates into metal solutions, e.g. of soluble nickel, cobalt or copper, a reducing agent and a chelant. Although substrates can be catalyzed with a variety of metals from Group 1B or Group 8, palladium, despite its cost, is often the catalyst of choice due to its activity. Maintaining adhesion of the catalyst to the surface is of considerable importance since loosely adhered catalytic metal can be washed from the surface in the agitation of the plating bath causing depletion of the metal value of the plating bath as uncontrolled metal deposition occurs, resulting in what is commonly referred to as a "crashed" bath.
Morgan et al. in U.S. Pat. No. 4,910,072 and O'Connor et al. in U.S. Pat. No. 4,900,618 disclose selective catalytic activation of catalytically inert polymeric films comprising complexes of a polymer and a compound of a Group 1B or 8 metal having essentially no metal on the surface. Solutions for preparing such films are prepared with a variety of organic solvents, e.g. tetrahydrofuran, acetone, methyl ethyl ketone, methanol, methyl acetate and ethyl acetate, which are preferably anhydrous. Low levels of water in the organic solvent can be tolerated provided the film is formed in a high humidity environment. Because of environmental concerns with organic vapor emissions, it is becoming more desirable to form films from aqueous solutions rather than from organic solvent solutions.
The use of aqueous coating systems to selectively provide catalytic surfaces has been a long term goal of practitioners in the field of electroless deposition of metals. Lenoble in U.S. Pat. No. 3,615,471 discloses the use of catalytic polymeric photoresistant coatings from aqueous solutions of a polymer such as polyvinyl alcohol (PVOH), a light activated crosslinker such as potassium bichromate and a catalytic metal compound such as palladium dichloride. Such coatings can be used to provide a catalytic pattern on circuit board substrate by exposing the dried coating to U.V. light through a template causing the exposed coating to crosslink; the unexposed water soluble coating is removed by water washing; and the crosslinked coating is cured, i.e. dried by heating at 190.degree. C. for 15 minutes, and electrolessly plated. Polichette et al. in U.S. Pat. Nos. 3,772,056; 3,772,078; 3,925,578; 3,959,547; 3,930,963; and 3,994,727 disclose selective electroless deposition of metal onto films coated from solutions of light or heat reducible metal salt solutions, e.g. containing cupric formate and a light sensitive reducing agent such as anthraquinone 2,6-disulfonic acid disodium salt and a minor amount of surfactant; after exposure to light through a pattern the unexposed coating is washed away with water allowing electroless deposition on the light reduced metal; and Polichette et al. in U.S. Pat. No. 3,779,758 disclose selective electroless deposition of metal onto light cured adhesive films coated from solutions of polymeric adhesive, a catalytic metal salt, e.g. palladium chloride, and a photosensitizer such as a diazonium compound; after exposure to light through a pattern the unexposed coating is washed away allowing electroless deposition on the reduced metal-containing, light-crosslinked adhesive. A disadvantage of using such coatings as disclosed by Lenoble and Polichette et al. is that inadvertent failure to wash away all of the non-crosslinked coating can cause undesired electroless deposition on the substrate or subsequent washing of the coating from agitation in the plating bath can cause the plating bath to crash, i.e. uncontrolled metal reduction and deposition throughout the plating bath.
Miller in U.S. Pat. No. 3,656,952 discloses a photosensitive polymeric, e.g. polyvinylpyrrolidone and polyoxyethylated fatty alcohol, coating containing a photoactive reducing agent, e.g. ferric ammonium oxalate, and a noble metal compound, e.g. complexes of palladium (II) and platinum (IV) with EDTA. On exposure to U.V. or visible light the light sensitive ferric ammonium oxalate causes reduction of the palladium compound. The film is then treated with a reducing agent such as dimethyl borane solution which reduces the palladium in the non-exposed surface. Electroless deposition of nickel results in a non-reversed reproduction of the image, e.g. because light-generated metal nuclei apparently decrease the quality of chemically reduced metal nuclei formed in the light-struck areas resulting in more or faster electroless deposition on the chemically reduced metal nuclei in the areas which were not exposed to light. A disadvantage of such coatings is a lack of good control in selectivity in electroless deposition between areas exposed to light and areas not exposed to light.
Yudelson et al. in U.S. Pat. No. 3,719,490 disclose light sensitive palladium compounds, e.g. potassium palladous chloride, palladium tetramine chloride and potassium palladium oxalate, which on exposure to actinic light form catalytic centers for the electroless deposition of metal; such compounds are applied as coatings from aqueous solutions or from solutions of a poymeric binder such as gelatin or PVOH with a boric acid crosslinker; and, after exposure to actinic light, metal can be electrolessly deposited on the light exposed surface. A disadvantage of using such coatings as disclosed by Yudelson et al. is that only surfaces which can be exposed to light can be provided catalytically active; for instance surfaces on fibers within a bundle which are not readily exposed to direct light cannot be made catalytic.
Rolker et al. in U.S. Pat. No. 3,900,320 discloses processes for metallizing plastic using films of polymer and a catalytic metal compound. In Example 5 Rolker et al. disclose preparing a film using a pre-plate solution of 0.05 parts of palladium chloride and 0.25 parts of polyvinyl alcohol in 100 parts of water; a polyester sheet was dipped in the pre-plate solution and air dried; and when the coated film was then dipped into an electroless nickel plating bath for three minutes, a layer of nickel was deposited. A disadvantage of using such coatings is that selective parts of a substrate can be made catalytic only by selective application of the coating.
Moreover, attempts to duplicate the experimental work of Rolker et al. have been generally unsuccessful. For instance, it has been discovered that when coatings of the Rolker et al. pre-plate solution are air dried at temperatures up to 100.degree. C., the solutions provide catalytically inert films, i.e. the films do not catalyze the electroless deposition of nickel. Only when films are dried at temperatures above the boiling point of water, e.g. at about 150.degree. C., are non-uniform catalytic films formed. Catalytic films formed from aqueous solutions at such high temperatures are undesirable because the films are often poorly formed with wide variations in catalytic activity, apparently because the rapid evolution of solvent causes concentration gradients of catalytic metal across the coating. This is manifest by colored spots on the dried film which comprise high levels of catalytic metal, e.g. about twice the concentration of catalytic metal as present on non-colored portions of the film. A disadvantage of using catalytic metal coatings prepared and dried according to Rolker et al. is the lack of uniform deposition of metal from electroless deposition solutions.
Metal-coated textile materials are useful for a variety of electromagnetic radiation shielding applications, e.g. wall coverings, gaskets, composite housings, protective clothing, and the like from adverse effects of electromagnetic interference. The shielding performance of such metal-coated textile materials is influenced by integrity, adhesion and electrical conductivity of the metal coating, especially the long term conductivity in common environments, e.g. metal-oxidizing environments of high humidity, rain, fog or salt spray.
Metal-coated textile materials prepared by electroless deposition techniques are preferred to textiles coated by other means, e.g. vapor deposition, because line of sight deposition from vapor does not provide uniform metal coating, e.g. at interior surfaces of the textile. Electroless deposition methods are generally more favored because catalytic metal such as palladium can be applied more evenly to fibrous surfaces even interior fibrous surfaces. Nishwitz in U.S. Pat. No. 4,002,779 discloses the metal coating of non-woven fabrics which are made catalytic to electroless deposition by sensitizing with an aqueous hydrochloric acid solution of tin chloride followed by treatment with an aqueous hydrochloric acid solution of palladium chloride. Copper-coated non-woven fabric prepared by such technique is reported to have surface resistivity of 30-80 ohms; and, nickel-coated non-woven fabric, 80-150 ohms.
More conductive metal coated textiles are disclosed by Ebneth in U.S. Pat. No. 4,201,825 where textile materials made catalytic using colloidal palladium are reported as having surface resistivity as low as 4 ohms for copper coatings and 10 ohms for nickel coatings. The environmental durability of the metal coatings was such that after only ten washings of 30 minutes duration in 30 C emulsifier-containing water, the surface resistivity of nickel-coated fabric degraded from 10 ohms to 300 ohms. In U.S. Pat. No. 4,572,960 Ebneth discloses metallized knitted polyester yarn catalyzed with a solution of butadiene palladium chloride in methylene chloride and plated with nickel having a resistance per square meter of 0.1-0.2 ohms; as indicated above the use of methylene chloride solutions are environmentally undesirable.